X-ray photo-induced atomic motion in Phase Change Materials and conventional covalent chalcogenide glasses

TL;DR

This study compares atomic motion in two similar chalcogenide glasses under X-ray irradiation, revealing distinct dynamic responses linked to their structural relaxation and phase change properties, with implications for material stability and behavior under irradiation.

Contribution

It provides the first detailed comparison of X-ray induced atomic dynamics in phase change and non-phase change chalcogenide glasses, highlighting different relaxation mechanisms and responses.

Findings

Ge15Sb85 exhibits immediate photo-induced yielding with stationary dynamics.

Ge15Te85 shows progressive slowing and a crossover from compressed to stretched exponential decay.

Temperature influences the dynamics, shifting from intrinsic stress relaxation to dose-controlled behavior.

Abstract

X-ray Photon Correlation Spectroscopy (XPCS) enables direct access to atomic-scale dynamics in disordered materials, revealing both spontaneous and X-ray-induced relaxation processes. Here, we study two compositionally similar alloy glasses near their glass transition temperatures: the phase change material (PCM) Ge15Sb85 and the non-PCM alloy Ge15Te85. Both exhibit X-ray induced atomic motion, yet with markedly different responses. Ge15Sb85 undergoes an immediate transition to a photo-induced yielding state, characterised by stationary dynamics governed solely by the absorbed dose. In contrast, Ge15Te85 shows a progressive slowing-down of the relaxation process, accompanied by a crossover from compressed to stretched exponential decay in the density autocorrelation functions. This behaviour is consistent with the emergence of liquid-like collective motion as supported by de Gennes narrowing in the wave-vector dependence of the dynamics at length scales comparable with the first sharp diffraction peak. Unlike Ge15Sb85, this alloy does not reach a stationary regime within experimental timescales, implying that the yielding transition occurs only after thousands of seconds with the available dose rate. Its response is also temperature dependent: at lower temperatures, the dynamics reflects intrinsic stress relaxation processes, whereas at higher temperatures becomes dose-controlled. These findings demonstrate that the dynamical response to X-ray excitation is not determined solely by chemical composition or bonding character, but results from the interplay between irradiation effects and structural relaxation pathways.